Canadian supermarkets have become the first in the world to stock genetically modified fish, and about five tonnes of GM salmon have been sold in the country in recent months.

The sales figure was revealed in the most recent earnings report of the US-based AquaBounty Technologies, whose hybrid Atlantic salmon – which contains a gene from a Chinook salmon and a promoter sequence from the ocean pout – has been at the heart of a heated debate over transgenic animals as food.

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Originally developed by a group of Canadian scientists at Newfoundland’s Memorial University, the salmon can grow twice as fast as conventionally farmed Atlantic salmon, reaching adult size in some 18 months as compared to 30 months. The product also requires 25% less feed to grow to the size of wild salmon, and could have a carbon footprint of up to 25 times less, the company has claimed.

In recent months, approximately five tonnes of GM salmon has been sold in Canada at a market price of US$5.30 per pound, AquaBounty said in a statement. “The sale and discussions with potential buyers clearly demonstrate that customers want our fish, and we look forward to increasing our production capacity to meet demand,” said Ronald Stotish, the company’s CEO.

As news broke that several tonnes of GM salmon had already been sold in Canada, organisations ranging from consumer rights groups to environmentalists reignited the call for mandatory labelling of GM foods in Canada. “The first genetically modified animals have arrived in the market and Canadian consumers are becoming, unwittingly, the first guinea pigs,” said Thibault Rehn of the group Vigilance OGM.

“The company did not disclose where the GM salmon fillets were sold or for what purpose, and we’re shocked to discover that they’ve entered the market at this time,” said Lucy Sharratt of Canadian Biotechnology Action Network.

Two supermarket chains – IGA Quebec and Costco – have announced they will not stock GM salmon. Others welcomed the news, describing it as a breakthrough for a growing industry.

After more than two decades in regulatory limbo, AquaBounty’s salmon was approved for sale in Canada in 2016, paving the way for it to become the first genetically engineered animal to enter the food supply. Canadian officials said four years of testing had found the modified salmon to be as safe and nutritious as conventional salmon, meaning no special labelling would be required to sell it.

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AquaBounty did not respond to an interview request from the Guardian or to questions regarding where in the Canada it had sold its salmon.

Regulators in Canada pushed forward, despite fierce opposition. While the company claimed that GM fish could help curtail the overfishing of Atlantic salmon and lessen the pressure on stocks of wild salmon, environmentalists worried that the transgenic fish – which is an Atlantic salmon with a growth hormone gene from a Pacific salmon as well as a gene from the ocean pout to promote year-round growth – could pose a risk to wild salmon populations.

The company has said its fish are sterile and are currently only being raised in landlocked tanks. Those sold in recent months in Canada are thought to have been bred in tanks in Panama, but the company is currently laying the groundwork to expand its operations to Prince Edward Island on Canada’s east coast.

Some 30 other species of GM fish – including tilapia and trout – are in development around the world, as are GM cows, chickens and pigs.

• This article was amended on 9 August 2017 to clarify a previous version of the headline, which incorrectly implied that shoppers knowingly sought out GM salmon.